Recent studies reveal that some patients with familial ALS (FALS) have a mutation of the gene that codes for the enzyme cu/zn superoxide dismutase (SOD). Although the mutation has not been demonstrated in sporadic cases, the striking phenotypic similarity between FALS and sporadic ALS suggests that a common pathogenesis may be present. Studies not yet reported in final form demonstrate that SOD is reduced by 50% in red blood cell lysates and lymphoblastoid lines and markedly reduced in the spinal cord anterior horn in pts with the mutation.